1. Field of the Invention
The present invention relates to a power amplifier, and particularly relates to a power amplifier for amplifying audio frequency signal or other analog signals, and a bridge circuit in a power amplifier.
2. Description of Prior Art
As shown in FIG. 1, a conventional power amplifier includes a comparator 1, a bridge circuit 2, and a low-pass filter 3. The bridge circuit 2 includes NMOS transistor 21 and PMOS transistor 22. The gates of NMOS transistor 21 and PMOS transistor 22 are both connected to the output end of the comparator 1. The source electrode of PMOS transistor 22 is connected to a high voltage signal HV, while the source electrode of NMOS transistor 21 is connected to ground. The bridge circuit 2 includes NMOS transistor 21 and PMOS transistor 22. The drains of NMOS transistor 21 and PMOS transistor 22 are both connected to the input end of the low-pass filter 3. During working, the comparator 1 compares a first analog signal (for example, a audio frequency signal) with a reference signal 5, and exports a square wave signal. The bridge circuit 2 realizes amplifying the audio frequency signal by activating the NMOS transistor 21 or the PMOS transistor 22, according to the square wave signal. The output signal of the bridge circuit 2 is changed to an audio frequency signal (a second analog signal) by the low-pass filter 3, and then exported to loudhailer 4.
Detailed descriptions regarding an audio power amplifier are disclosed in other documents such as Chinese patent application No. 200810166133.0 and No. 200810239524.0. The audio power amplifier disclosed in patent application No. 200810166133.0 includes a comparator and a full bridge output circuit, and the comparator exports a pulse width modulated square wave signal. The full bridge output circuit includes an inverter and four MOS transistors M1, M2, M3 and M4, which amplifies the pulse width modulated square wave signal exported by the comparator. The audio power amplifier disclosed in patent application No. 200810239524.0 includes a pre-amplifier, an error amplifier, a comparator, a bridge circuit and a feedback circuit, wherein the bridge circuit and the pre-amplifier are composed of transistors.
The circuits described above are all started by pulse signals activating the corresponding transistors. As we all know, transistors cost much power during working. What's more, manufacturing comparators and triangle wave generators which generate the reference signal 5 adopts standard CMOS technology, while manufacturing PMOS and NMOS transistors adopts high voltage CMOS technology which is different from the standard CMOS technology. Therefore the comparators, triangle wave generators, PMOS and NMOS transistors mentioned above all have to adopt high voltage CMOS technology, which is complex, enlarges the area of all devices and increases the cost.